Sunlight could cool an atom to its coldest possible temperature

Sunlight could be used to cool a charged atom to its lowest temperature allowed by the laws of physics.

In the 1990s, several people won the Nobel Prize for training how to make atoms extremely cold with precisely controlled laser light. Now Amanda Younes and Wesley Campbell at the University of California, Los Angeles, found that parts of the cooling process could be done with light coming directly from the sun.

The researchers calculated how a single positively charged barium atom, or ion,…